This invention relates to polymeric compositions and processes particularly useful in protective surface paint coatings based on certain reactive emulsion polymers adapted to be self-curing latex binders. The self-curing polymers include copolymerized functional monomers (except amine) in conjunction with alkyloacrylamide monomer to render the polymer self curing. The self-curing or self-reacting polymer is ion exchanged to render a substantially improved ambient curing, self-crosslinking polymer.
Functional latexes are known such as suggested in U.S. Pat. No. 3,991,216 based on interpolymers of copolymerized acrylamide monomer, carboxylic monomer, and other ethylenically unsaturated monomers. Other patents disclosing similar latexes are U.S. Pat. No., 2,978,434 and U.K. Pat. No. 2,034,334, which suggest that premature condensation of emulsion copolymerized alkylol acrylamide derivatives with functional monomers having carboxyl, hydroxyl, or similarly reactive functional groups can be eliminated by copolymerization in two stages. These references teach that first stage monomers contain no alkylol acrylamide monomer and are copolymerized under acidic conditions at 70.degree.-80.degree. C., but second stage monomers containing an alkylol acrylamide together with other functional monomers containing hydroxyl or carboxyl group are copolymerized under neutral or slightly alkaline conditions at temperatures no greater than 50.degree. C. The disclosed two-stage processes avoid condensation reactions until the coating is baked as a film onto the desired substrate. Similarly, Journal of Coatings Technology, Vol. 51, No. 657 (October 1979) discloses low molecular weight acrylamide latexes adapted to crosslink upon curing as a film.
U.S. Pat. No. 4,499,212 discloses a high molecular weight two-stage thermosetting acrylic emulsion copolymer containing crosslinked surface polymer chains and carboxylic and methylol acrylamide functionalities particularly useful in the formulation of coatings. Reactive self-curing latexes containing copolymerized monomers including functional carboxyl, hydroxyl, amine or amide monomers in combination with alkylol acrylamide monomers is polymerized in a stepwise reaction to concentrate the alkylol acrylamide on the surface of the latex polymer particles. The second stage polymerization takes place at temperatures above about 70.degree. C. wherein a minor amount of alkylol acrylamide reacts with a minor amount of functional monomer during the addition polymerization of ethylenic monomers to produce crosslinked polymer chains in the polymer surface.
It now has been found that stabilized self-curing latexes containing copolymerized monomers comprising reactive carboxyl and hydroxyl monomers in combination with alkylol acrylamide monomers, but substantially free of amine monomers provide a self-curing thermosetting latex. In accordance with this invention, the self-curing latex is ion exhanged to obtain a low pH, preferably below 2.5, to provide an excellent self-curing thermosetting latex that can be heat cured or ambient cured without external cross-linking agents. It has been found that ambient dry consumer trade sales latex paint compositions (wall and ceiling paints, etc.) can be formulated to produce highly desired air dry films which further crosslink over a period of time to produce substantially improved film integrity properties. Prior to this invention, conventional air dry consumer paints dried by evaporation of water and subsequent coalescence of binder polymer particles, but such binders were merely coalesced and not crosslinked. In said commonly assigned Ser. No. 100,537, reactive emulsion polymers are crosslinked with glycoluril. In accordance with this invention, the binder polymer particles coalesce but then further crosslink at ambient room temperature or heat cured without the need for external crosslinker such as melamine or glycoluril. Upon the passage of several days, a substantially improved crosslinked film is formed. Elimination of crosslinkers substantially reduces or eliminates undesirable formaldehyde emissions. Hence, excellent room temperature cured consumer paints can be produced to be soft enough to provide excellent film formation and will subsequently crosslink at ambient room temperature to form a cured thermoset paint film exhibiting superior film integrity properties.
In accordance with this invention, an ambient or heat cured latex paint free of melamine or glycoluril-type crosslinkers and coreactive binder polymers can be produced by using conventional surfactants, where the mixture is subsequently subjected to cationic exchange to remove the cations from the anionic surfactant as well as from other sources. The emulsion binder polymer specifically contains hydroxyl and/or carboxyl groups as well as alkyl acrylamide groups, but not amine groups. Amine groups block reactivity, inhibiting cure and render the ionic exchange process of this invention inoperative. The resulting cationic exchange step produces a clear emulsion mixture adapted to cure at room temperature without the addition of acid catalysts such as p-toluenesulfonic acic, and hence, the disadvantages associated with the use of external acid catalysts are overcome by this invention. Dynamic mechanical analysis further demonstrates that the ambient cure achieved by ion exchange is faster than by acid catalysis. The pigmented or clear mixturesexhibit excellent uninhibited cure along with long-term viscosity stability. The cation exchange treatment of the self-curing latex is stable against settling and exhibits no flocculation. The ion-exchange process utilized a proton-substituted cation exchange resin to remove cations which unexpectedly yields an ambient cure liquid coating with long term package stability as compared with the acid-catalyzed liquid coating which reacts quickly but typically destabilizes and gels in 1-5 days at room temperature. The improved clear or pigmented emulsion coatings of this invention are indefinitely stable at room temperature yet, upon drying, thermoset at room temperature to give a fully crosslinked film. These and other advantages of the invention will become more apparent by referring to the specification and illustrative examples.